1. Field of the Invention
This invention relates to a method of making a sheet polarizable electrode for an electric double layer capacitor from a material containing a carbon powder, a conductive assistant and a binder.
2. Description of the Related Art
Electric double layer capacitors have a large capacity and an excellent charge and discharge cycle property. Therefore, the electric double layer capacitors are used for the purposes of backup power supplies for various electronic devices and batteries for means of transportation such as automobiles. Alternatively, applications of the electric double layer capacitors to the aforesaid appliances have been under examination. Particularly when used as batteries for transportation means such as automobiles, the electric double layer capacitor is required to have a large electrostatic capacity or capacitance. A long sheet polarizable electrode is required in such an electric double layer capacitor as having a large electrostatic capacity. Various methods have been proposed for making a long sheet polarizable electrode.
JP-A-2001-307964 discloses the following method of making a sheet polarizable electrode for an electric double layer capacitor, for example. Firstly, an aqueous solution made by mixing isopropyl alcohol with demineralized water is added to activated carbon to be mixed together by a mixer. Carbon black and fluorocarbon resin are then added to the obtained mixture to be further mixed together by the mixer, whereupon a material or a mixture is obtained. The mixture is then kneaded by a kneader. The kneaded material is dried and thereafter, pulverized so that a grain diameter is not more than 2 mm. The pulverized grain is rolled between parallel rollers into a sheet-shape. The obtained sheet polarizable electrode has an average thickness of about 200 μm and a density of 0.70 g/cm3.
In the foregoing method, however, the material cannot be distributed uniformly in the mixing step such that a failure is easy to occur in the mixing. Upon occurrence of a failure in the mixing, the material cannot be distributed uniformly in the kneading step, either. As a result, when the material is formed or rolled into a sheet shape, segregation tends to easily occur in the material. Upon occurrence of segregation, the effect of a binder becomes unstable, whereupon ends of a formed sheet are cracked, or cavity or the like occurs in the formed sheet. Consequently, the quality of the formed sheet is unstable and a necessary strength cannot be obtained.